West Florida shelf and slope prime target for gas, oil in eastern Gulf of Mexico

June 11, 2001
The Gulf of Mexico off Florida is one of the nation's last remaining petroleum frontiers.

The Gulf of Mexico off Florida is one of the nation's last remaining petroleum frontiers. Some 80,000 sq miles of state and federal area covered by the sea to a depth of less than 3,300 ft are so far only marginally explored for oil and gas.

The discovery of potentially recoverable giant natural gas reserves of 2.6 tcf off the Florida Panhandle, confirmed in 1995, rekindled Florida's objection to petroleum activity in federal waters off its coast under Gov. Jeb Bush in response to public fear of adverse impacts.

It has become Florida's policy to oppose oil and gas operations within 100 miles off its shore. Pres. Bill Clinton blocked further leasing after Lease Sale 181 in these federal waters until 2012.

The new administration under President George W. Bush has developed an energy plan that calls for greater domestic oil and gas production, in particular an increase in gas supply. The West Florida shelf and slope is the prime target for such an endeavor.

It will be in the national interest to identify areas where more commercially attractive volumes of oil and gas can be detected off Florida, to determine whether the deposits will be gas, gas-condensate, or light, medium, or heavy oil, and to consider these accumulations' chances of discovery.

The US Minerals Management Service (MMS) attributed in its assessment mean undiscovered, recoverable petroleum resources of more than 2 billion bbl of oil and 8 tcf of gas to the eastern Gulf of Mexico (Mesozoic province) that encompasses the West Florida shelf and slope and part of the Mississippi-Alabama shelf.1

Exploration history

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The location of the wells drilled on the West Florida shelf and slope and nearby land, including on the Florida Keys, and the places of discovered commercial and noncommercial oil and gas accumulations or significant shows demonstrate how sparsely the West Florida shelf and slope has been explored (Fig. 1).

Few petroleum companies have intermittently explored in Florida state waters, an area of about 8,000 sq miles, during 1947-68 and again in 1983. Twenty-three exploratory wells were drilled in state waters during that time. Florida closed its coastal waters to oil and gas activities in 1990.

In federal waters off Florida, of which about 72,000 sq miles are covered by the sea to a depth of less than 3,300 ft, the MMS had issued during 1959-88 a total of 338 lease tracts equaling 3,042 sq miles in area. Today, only 110 of those leases covering 990 sq miles are still active.2 Thirty-nine wells were drilled in federal waters in the course of exploration and appraisal.

Sunniland heavy oil field was discovered in South Florida in 1943. Development of that trend lasted until 1985. Oil shows were observed in the Florida Keys in 1947-60, with one well testing heavy oil (Permit 275).3

Flomaton field in Alabama and giant Jay light oil field in the western Florida Panhandle were discovered in 1968 and 1970, respectively. Several other significant gas-condensate and light oil fields were found subsequently on that trend. The last field was discovered in Florida in 1986.3 4 5

Oil stains were observed in two wells in the Florida Panhandle near Panama City and Apalachicola (Permits 746 and 814).6

Some gas and heavy oil accumulations were discovered in federal waters off Alabama at an intermediate level on Main Pass blocks 221 and 253 in 1972 and proved to be noncommercial.7 Commercial gas was found lately in the same region somewhat deeper on Viosca Knoll blocks 69 and 252 and Mobile Block 991.8

A deep dry gas field, was detected at the mouth of Mobile Bay in 1979.9 Mary Ann field has developed into a trend with several prolific dry gas fields10 and now extends into federal waters.11

Shallow gas was discovered in and off Alabama, including in federal waters, since the 1980s.12 13

In West Florida shelf federal waters, petroleum was observed off the Florida Panhandle between the projected border to Alabama and Panama City at four places. A noncommercial oil accumulation was found on the Destin Dome in 1985 (Destin Dome Block 160-1). Two years later, noncommercial gas and condensate were discovered a short distance to the west (Destin Dome Block 111) and nearby to the northwest (Pensacola Block 948-1, -2). Another well drilled in 1988 in the same region penetrated 22 ft of oil-bearing reservoir that was never tested (Pensacola Block 996).1

One well off the southern Florida Peninsula (Charlotte Harbor Block 672) encountered oil shows in 1981.1

The offshore federal lease blocks on which those noncommercial accumulations were registered have been relinquished.

A dry gas find off the Florida Panhandle in 1987 was verified in 1995 after two successful appraisal wells to be a giant field (Destin Dome Block 56 Unit).1 The importance of that field, besides its volume, lies in the fact that it extends the Mobile Bay Deep Gas trend at least 40 miles to the east, to offshore Florida.

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The stratigraphic position of the established petroleum sources and accumulations (fields and important shows) is depicted on Fig. 2.

Plays, resources

Four plays can bear on the oil and gas resource potential off Florida. The Norphlet, Smackover, and Sunniland plays are regionally established gas, condensate, or oil producers. Oil has been tested from the Dollar Bay play in one well.

MMS recognizes in its assessment of the Gulf of Mexico's Mesozoic province in federal waters four, partly differently defined plays. MMS ascribed the following estimates of mean undiscovered, recoverable resources:1

  1. Norphlet play: 591 million bbl of oil and 7.121 tcf of gas in a prospective, nearshore belt on the northern West Florida shelf from Alabama to Apalachicola.
  2. Smackover play: 29 million bbl of oil and 48 bcf of gas in the same belt;
  3. Bone Island-Pumpkin Bay-Brown dolomite-Sunniland plays: 419 million bbl of oil and 31 bcf of gas on the perimeter of a basin on the southern West Florida shelf;
  4. Bone Island-Pumpkin Bay-Brown dolomite-Sunniland plays: 587 million bbl of oil and 42 bcf of gas in the center of that basin.

Four amendments to MMS's assessment are proposed here:

  1. To include in the Norphlet and Smackover plays the territory of the continuation of the prospective belt from the northern to the central West Florida shelf and slope, from Apalachicola southward to off Tampa, and
  2. To attribute to the basin on the southern West Florida shelf with two Sunniland plays a lower petroleum potential than currently assigned.
  3. To include the Dollar Bay play, which had tested heavy oil from a noncommercial accumulation (Permit 222) and was not included in MMS' assessment; and
  4. To discount the Bone Island play for reason of carrying a high exploratory risk, and to classify the Pumpkin Bay, a potential oil source, and the Brown dolomite play as being hypothetical. Both plays contain oil stains.

The other plays considered by MMS, like the James and Andrew plays, carry a high exploratory risk and are therefore discounted here.

The seismic-stratigraphic interpretation of the offshore region south of Apalachicola by Dobson and Buffler14 vaguely defines the continuation of the combined Norphlet and Smackover plays from Apalachicola to off Tampa. The seaward limit of the prospective belt in this area is drawn here arbitrary.

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Considering the regional distribution of the lower Smackover oil source from onshore Alabama to Apalachicola, it is safe to assume that the source continues from Apalachicola southward. Based on the increasing thermal maturity of oil source material and generated petroleum with increasing depth, areas are delineated with a dry gas, condensate, and light to medium oil potential in Norphlet and possibly Smackover plays on the northern and central West Florida shelf (Fig. 3).

Traps for petroleum can be provided in that additional region by basement structures as they occur in onshore Alabama15 and possibly also by post-depositional faulting comparable to the Pollard-Foshee fault trend in Alabama,16 though most likely not salt-induced. Stromatolite mounds17 have not been established as significant Smackover petroleum-bearing stratigraphic traps.

Chances are better than average to discover in the Norphlet play more dry gas, condensate, and light oil accumulations of a commercial volume from the offshore projected Alabama state line to off Tampa. MMS' current assessment of recoverable resources of 591 million bbl of oil and 7.121 tcf of gas must be increased considerably when taking into account the increase in prospective area.

The prospectivity of the upper Smackover play from the offshore projected Alabama state line to off Tampa appears to be low. No significant increase of the Smackover potential beyond MMS' estimate of 29 million bbl of oil and 48 bcf of gas, even with the addition of prospective area, can currently be advocated.

The southern Florida Peninsula yields the one heavy oil-producing Sunniland play on the Sunniland trend. The subtle stratigraphic traps-reservoirs of the trend were charged with oil because of the optimal development of an internal, underlying oil source around the site of the traps-reservoirs.18 The source is thermally early mature.

A test of the Sunniland play on the Florida Keys flowed heavy oil,3 and oil was logged in the Sunniland play in one well on the southern West Florida shelf.1

Heavy oil was tested from the overlying Dollar Bay play around the Sunniland trend (Permit 222),18 19 and many more oil shows were registered.20 But no significant reservoir development has been noted anywhere in that interval. The play yields another, less thermally mature, internal oil source. The onshore petroleum habitat serves here as an analog for the assessment of the adjacent offshore region.

The Sunniland and Dollar Bay plays are buried to greater depth than onshore in a basin on the southern West Florida shelf. The top of the Sunniland play is expected to occur offshore at 14,000-14,500 ft,21 i.e., about 2,500 ft deeper than on the producing onshore trend. Therefore, any existing oil source is expected to be offshore more thermally mature than onshore.

A potential source in the Sunniland play can have reached theoretically the middle stage of thermal maturation and a Dollar Bay source the full early stage of thermal maturation. The quality of generated oils would improve to middle grade for the Sunniland play and remain heavy grade for the Dollar Bay play (Fig. 3). The two plays ascend toward the Florida Escarpment, where they outcrop subaqueously and are therefore flushed.

The chances of discovering commercial fields in the Sunniland or Dollar Bay plays on the southern West Florida shelf are lower than average. It will be difficult to duplicate offshore the same setting of stratigraphic Sunniland reservoirs and low-relief traps with the same coincidental oil charge as onshore and to find economic reservoirs in the Dollar Bay play.

MMS's resource estimate of a total of 1.006 billion bbl of oil and 73 bcf of gas on the southern West Florida shelf from the two combined Sunniland plays, the here-discounted Bone Island play, and the hypothetical Pumpkin Bay and Brown dolomite plays must be reduced when considering that a total of only 10l million bbl of oil and 9 bcf of gas have been produced onshore since 1943 with only few reserves remaining now.

The oil and gas-bearing Cenozoic province of the central Gulf of Mexico, encompassing the Mississippi Delta slope and the Mississippi-Alabama shelf, terminates along the offshore projected boundary to Alabama and has, therefore, no effect on the resource potential off Florida (Fig. 3).

It must be noted that condensate and light oil, as they can be found on the northern and central West Florida shelf and slope, are highly volatile (gasoline-like) liquids and will, therefore, evaporate fast as a possible pollutant. Heavy and medium oils, as they can be detected on the southern West Florida shelf, are less volatile, will therefore dissipate more slowly, and can cause pollutant environmental impacts.

Proposed Lease Sale 181 (Figs. 1 and 3) covers an area that coincides with the DeSoto high off Florida. On that high, basement subcrops at a depth close to 19,000 ft.14 The existence of a fringe of prospective Norphlet reservoirs and an oil-generating Smackover source around basement is hypothetical. A well on DeSoto Canyon Block 512 in the deferred leasing area bottomed at 12,300 ft and did not reach the depth of such a deeper play.

References

1. Lore, G.L., Ross, K.M., Bascle, B.J., Nixon, L.D., and Klazynski, "Assessment of conventionally recoverable hydrocarbon resources of the Gulf of Mexico and Atlantic Outer Continental Shelf as of Jan. 1, 1995," Minerals Managernent Service-Gulf of Mexico Region, OCS Report 99-0034 (CD-ROM), 1999.

2. US Minerals Management Service, www.gomr.mms.gov

3. Applegate, A.V., and Lloyd, J.M., "Summary of Florida petroleum production and exploration, onshore and offshore, through 1984," Florida Geological Survey Information Circular 101, 1985, 66 p.

4. Bolin, D.A., Mann, S.D., Burroughs, D., Moore, H.E., Jr., and Powers, T.J., "Petroleum atlas of southwestern Alabama," Geological Survey of Alabama, Atlas 23, 1989, 218 p.

5. Lloyd, J.M., "1995 Florida petroleum production and exploration," Florida Geological Survey Information Circular 111, 1997, 62 p.

6. Applegate, A.V., Pontigo, F.A., Jr., and Rooke, J.H. "Jurassic Smackover oil prospects in the Apalachicola embayment," OGJ, Jan. 23, 1978, pp. 80-84.

7. Burgess, G.L., Peterson, R.H., Bascle, B.J., and Nixon, L.D., "LKI B.1. Lower Cretaceous shelf-margin carbonate," in Seni, S.J, Hentz, T.F., Kaiser, W.R., and Wermund, E.G., Jr., eds., "Atlas of northern Gulf of Mexico gas and oil reservoirs, Vol. 1, Miocene and older reservoirs," Bureau of Economic Geology, University of Texas at Austin, 1997, pp. 173-175.

8. Petty, A.J., "Northeast Gulf's James, Andrew exploration history outlined," OGJ, Nov. 1, 1999, pp. 94-101.

9. Marzano, M.S., Pense, G.M., and Andronaco, P., "A comparison of the Jurassic Norphlet formation in Mary Ann field, Mobile Bay, Alabama, to onshore regional Norphlet trends," GCAGS Transactions, Vol. 33, pp. 85-100, 1988.

10. Alabama State Oil & Gas Board, www.ogb.state.al.us

11. Kugler, R.L., Mink, R.M., Burgess, G.L., Peterson, R.H., Bascle, B.J., and Nixon, L.D., "UU A.l. Upper Jurassic aggradational sandstone," in Seni, S.J., Hentz, T.F., Kaiser, W.R., and Wermund, E.G., Jr., eds., "Atlas of northern Gulf of Mexico gas and oil reservoirs, Vol. 1, Miocene and older reservoirs," Bureau of Economic Geology, University of Texas at Austin, 1997, pp.8-10.

12. Smith, C.C., and Mink, R.M., "Upper Miocene Dauphin natural gas sands in offshore Alabama," GCAGS Transactions, Vol. 47, 1997, pp. 541-547.

13. Smith, C.C., and Mink, R.M., "Middle and Upper Miocene natural gas sands of onshore and coastal Alabama," GCAGS Transactions, Vol. 48, 1998, pp. 417-422.

14. Dobson, L.M., and Buffler, R.T., "Seismic stratigraphy and geologic history of Jurassic rocks, northeastern Gulf of Mexico," AAPG Bull., Vol. 81, 1997, pp. 100-120.

15. Mink, R.M., and Mancini, E.A., "Upper Jurassic and Lower Cretaceous oil reservoirs of the updip basement play: Southwest Alabama," GCAGS Transactions, Vol. 45, 1995, pp. 441-448.

16. Tew, B.H., Mink, R.M., Mann, S.D., and Bearden, B.L., and Mancini, E.A., "Geologic framework of Norphlet and pre-Norphlet strata of the onshore and offshore eastern Gulf of Mexico," GCAGS Transactions, Vol. 41, 1991, pp. 590-600.

17. Baria, L.R., Stoudt, D.L., Harris, P.M., and Crevello, P.D., "Upper Jurassic reefs of Smackover formation, United States Gulf Coast," AAPG Bull., Vol. 66, 1982, p. 1,449-82.

18. Applegate, A.V., and Pontigo, F.A., Jr., "Stratigraphy and oil potential of the Lower Cretaceous Sunniland formation in South Florida," Florida Geological Survey Report of Investigation 89, 1984, 40 p.

19. Mitchell-Tapping, H.J., "New exploration play in Florida-The Fredericksburg Dollar Bay formation," GCAGS Transactions, Vol. 40, 1990, pp. 607-621.

20. Winston, G.O., "The Dollar Bay formation of Lower Cretaceous (Fredericksburgian) age in South Florida; its stratigraphy and petroleum possibilities," Florida Bureau of Geology Special Publication 15, 1971, 99 p.

21. Lore, G.L., written communications, 2000.

The author

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Klaus H. Gorbandt is a retired geologist living in Gulf Breeze, Fla. He held assignments with increasing responsibilities in oil and gas exploration with Mobil in Austria and Libya in 1957-70, Deminex in Germany and Nigeria in 1970-75, OMV in Austria in 1975-81, and Gulf-Chevron Overseas in 1981-95. He has a PhD degree in geology from the University of Vienna. E-mail: [email protected]